X-rays and neutrons are an effective probe medium for evaluation of many features of materials. X-ray diffraction, spectrometry and microscopy are widely used for materials structure and composition measurements. Many applications require an x-ray beam having controlled beam characteristics in its interaction with the target, and some of them need an x-ray optic for an analysis of the beam after its interaction with the material sample. A variety of x-ray optics is in use in analytical instruments.
One of the x-ray optics that is in quite wide use is a glass polycapillary optic. The glass polycapillary optic comprises a bundle of hollow glass fibers arranged in some specific way. These hollow fibers guide x-rays entering them, and by total external reflection, the x-ray beam is transmitted along the fiber. Various polycapillary optic designs exist for focusing an x-ray beam, collimation, and the like.
A polycapillary optic can be used in a wide range of x-ray photon energies, but in the range of laboratory photon energies (5 to 25 keV) it has various benefits. It can provide a uniquely large geometrical solid collection angle up to several tens of steradians, which provides an efficient use of the x-ray energy emanating from the source. Further, the polycapillary optic can shape the beam in three dimensions.
These two features make polycapillary optics a preferred component of x-ray optics. An alternative arrangement can be provided with a crystal bent in two planes, but its capturing capabilities are lower due to limitations of angular acceptance in the diffraction plane of a Johann crystal.
A polycapillary focusing optic is preferably used in applications that do not require a high beam monochromaticity and can utilize a beam with a high convergence. Similarly, polycapillary collimating optics can provide a beam of large size and flux when requirements for the beam divergence are not too high.
Many analytical applications are suitable for polycapillary optics, and polycapillary optics are in wide use in micro x-ray fluorescence analyzers, x-ray diffractometers for stress and texture analysis, and many other applications.
The technology of glass polycapillary optic fabrication progressed significantly since their first use. A variety of optical arrangements and shapes can be produced. An opening lumen of a single capillary and the wall thickness can vary in a wide range, down to a micrometer-scale capillary diameter. On the other hand, a glass polycapillary optic is not free from some principal and technological limitations.
Capillary technology is currently limited to glass material formed in a high-temperature process. Currently, there is no technology available for a good control of mid and high spatial frequency roughness of the internal wall surface during this process. Absent a metrology for measuring the roughness directly, representative results of the roughness are currently obtained through modeling the optic with the different roughness parameters and comparing the results with experimental data. These results show that the internal capillary structure is not perfect. Further technology refinements may improve these parameters, but the internal surface roughness may affect optical efficiency well into the future.
A single capillary optic may be used as well, although not as universally as a glass polycapillary optic. Several typical shapes of single capillary optics are particularly common.
A straight single glass capillary is commonly used to form an x-ray beam with a predefined beam cross section and divergence. One advantage of a single straight capillary over a system with two pinholes is that the part of the beam that passes the first pinhole, but would miss the second pinhole, is retained in the optical path of the single capillary by total external reflection from the capillary walls. The mechanism of radiation penetration through the straight capillary is the same as for polycapillary systems, utilizing multiple total external reflections.
Similarly, conical single glass capillary can be used to concentrate x-ray radiation. A single total external reflection can be utilized in a capillary with a more sophisticated internal shape, for instance an ellipsoidal configuration. These kinds of optics are typically not made exclusively of glass, but include other materials.
One more specific device that provides x-ray beam with a width on a nanometer scale in one dimension is an x-ray waveguide. The coupling and propagation of radiation through this device is described in the terms of wave theory. The design condition could, however, be formulated in physical-geometrical terms for a three-layer symmetric structure: θCc>θCg, where θCc and θCg are critical total external reflection angles of outside cladding (θCc) and of guiding layers (θCg).
Similarly to the polycapillary glass optic, single capillary optics will benefit from a better surface precision and smoothness.